Degraded (converted) starches are desirably used or required in many industrial applications including, for example, in the manufacture of gypsum-board for dry wall construction, in paper and paper board manufacture, in textile warp sizing applications, as well as in the production of starch gum candy. It is known that undegraded native cereal and potato starches show a natural occurring variation in viscosity after gelatinization in water at concentrations of about 4-12% by weight. In some industrial operations, process control conditions require that a starch exhibits a reproducible particular defined viscosity after gelatinization. Mild conversion of the starch can be used to provide such required consistency.
The degradation of starches by conversion involves mainly a scission of the starch molecules to lower molecular weight fragments. When this is carried out on a granular starch, the granular structure is weakened and the granules tend to disintegrate more readily and more rapidly during the gelatinization process leading to a lower hot paste viscosity.
In commercial practice, starch is ordinarily converted by acid or enzyme conversion techniques. Acid conversion is preferred due to the ease in handling and recovery afforded by a granular starch as opposed to starch in dispersed form as necessitated by enzyme conversion.
In preparation of the converted starches by acid treatment, the granular starch base is hydrolyzed to the required viscosity in the presence of an acid, such as sulfuric or hydrochloric acid, at a temperature below the gelatinization point of the starch. The starch is slurried in water, and the acid (usually in concentrated form) is then added. Typically, the reaction takes place over an 8-16 hour period, after which the acid is neutralized with alkali (e.g., to a pH of 5.5), and the starch recovered by filtration.
In preparation of the converted starches by enzyme treatment, the granular starch base is slurried in water, and the pH is adjusted to about 5.6-5.7 with alkali or acid. A small amount of alpha amylase enzyme (e.g., about 0.02% on the starch) is added to the slurry, which is then heated above the gelatinization point of the starch. When the desired conversion is reached, the pH is adjusted with acid (e.g., to about 2.0) to deactivate the enzyme and the dispersion is held at the pH for a period of at least 10 minutes. Thereafter the pH may be readjusted. The resulting converted starch dispersion is usually jet-cooked to ensure complete solubilization of the starch and deactivation of the residual enzyme.
The use of hydrogen peroxide on starch as a converting (thinning) agent alone or together with metal catalysts has been known for some time. U.S. Pat. No. 3,975,206 assigned to A. E. Staley Manufacturing Company is directed to an improved process for oxidative thinning of starch employing hydrogen peroxide in combination with heavy metal salt catalysts such as iron, cobalt, copper or chromium. It is stated that substantially more effective thinning was obtained when no pH adjustments were made (pH about 3.2) as compared to a similar reaction maintained at pH 9.7 with sodium hydroxide. The 206 patent itself lists a number of references directed to degrading (thinning) starch with hydrogen peroxide under a variety of conditions.
U.S. Pat. No. 2,307,684 assigned to Buffalo Electro-Chemical Company teaches a method of liquefying starch by treating natural or raw starch with a peroxide in the presence of a catalyst which can be a metal such as copper or manganese or their salts. It is suggested that an initial pH of above 7, preferably about 9 to 10 be produced in the mixture, but the liquefied starch finally obtained after the reaction possesses a pH of between about pH 6 and pH 8. The liquefying treatment employs temperatures above the gel point of the starch.
U.S. Pat. No. 3,655,644 assigned to Grain Processing Corporation discloses a method of thinning derivatized starch using hydrogen peroxide employing a temperature in the range of 80.degree. to 130.degree. F. (27.degree. to 54.degree. C.), a pH in the range of 7.0 to 12.0, and a copper ion catalyst in an amount from about 5 to 100 parts per million by weight of starch.
T. Omori, in an article in the Journal of Biochemistry, Volume XIV:2, pages 331-337 (in German), reports on peroxide treatment of gelatinized starch employing various heavy metals, including manganese chloride. Typical reactions were run at pH 3.6 on starch employed in very low concentrations, about 0.05%.
It is seen that there is still a need for an efficient, commercially useful process for controllably preparing a degraded granular starch which process does not employ acids or enzymes.